High fidelity receiver



March 26,- 1940. R KIRKWQOD 2.194 558,

HIGH FIDELITY RECEIVER Filed Aug. 31, 1938 ,2 ShetsSheet l 3nnemor March 1 L. R. KIRKWOOD 2,194,558

HIGH FIDELITY RECEIVER Filed Aug. 31, 1938 2 Sheets-Sheet 2 59 & $3

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Junentor dttorneg Patented Mar. 26, 1940 ZSW HIGH rrnsnrry Rnomvsn Loren R. Kirkwood, Oaklyn, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application August 31,

, 17 Claims.

The present inventionrelates to radio receiving systems, and has for its primary object, to provide 'means for tuning a radio receiving system through a predetermined frequencyrange with substantially uniform selectivity, whereby the iidelity in'signal reception may be made more uniform throughout the tuning range, and otherwise enhanced; 1

m In modern receivers, this involves invention, to provide improved means for tuning,

two or more radio frequency or. signal receiving circuits whereby the overall'tuning response characteristic or band widththereof may be subs stantially constant throughout a relatively Wide tuning range.

In radio receivingsystems heretofore employed,

it has been considered necessary to further increase the" selectivity of the radio frequency or tunable signal circuits at the low frequency end of the tuning range, and to further broaden the I selectivity at the high frequency end of the tuntuning inductances are made substantiallyidem tical for the same tuning range and as the sec ondaries of interstage couplingtransforiners, are

coupled with primaries of low inductance'forhigh frequency response that resonate above thetuning range.

Such tuning systems provide a variable band width in the tuning response characteristic which is relatively narrow at the low frequency quency end of the tuning range conformity with the selectivity and broaolness or the tuning determined for the high frequency end of the tuning range, thereby to maintain substantially 55 constent band width in the frequency response the tuning of two, or more signal receiving or radio-frequency circuits through the same frequency range, and

ing range. In such systems, furthermore, all

tuning or reduce the selectivity at the low ire 1938, Serial No. 227,791

((3l.250-.-20)v l characteristic throughout thetuning range of the receiving system. l 1 1 1 Ina receiving system for Whichjthe present invention is particularly adapted, thetwo cascaded tunable circuits may comprise an antenna or R.- F". input circuit {connected withgafirst Eh-F. amplifier, and a second RQ-F. circuitcoupled tov the output circuit of the first R'.-F, amplifier and connected with the first detector. known, the selectivity of such circuits increases as the frequency to which they are tuned is re duced. Each tunable circuit mayinclude the secondary inductance of an R.-F. transformer and may be provided with 'a variable shunt tuning capacitor section, the various sections being 0on trollable in unison witha section for tuning the oscillator. p 1 i It is also a still further object of the present invention to provide means for mistuning the signal receiving circuits by gradual change in alignment 1 at the low frequency end of the tuning range and for restoring the alignment of said circuits at the high frequency endthereof, therebyto improve the band width response throughoutthe tuning undesired signals in the tuning system adapted for substantiallyuniform selectivity over a relatively wide frequency'range; and an improved circuit arrangementthereforq y In carrying out the above andother objects of 35 the present invention, and differing from present practice, the radio frequency and detector tuned circuits include unequal inductances, preferably provided by the secondary windings of suitable R.-F. coupling transformers, the primary induc- 0 tances of which are equal and relatively high to resonate below the tuning range of the secondaries. The unequal tuning inductances are tuned by a gang or uni-control variable capacitor having sections of substantially equal capacity and variable capacity range, together with a suitable shunt trimmer capacitor for each section.

In general, the circuit arrangement for providing substantially uniform selectivity includes a plurality of tunable signal circuits in cascade relation to each other, each circuit including induc-I tive and capacitive reactance elements. Means are provided for varying all-of the capacitive re actance elements or all of the inductive rea ct'ance "elements conjointly as by uni-control means, the 5s As is well 16 inductive reactance elements being of unequal inductance value when the capacitive reactance elements are made variable, and the capacitive elements being of unequal capacity value when the inductive reactance elements are made variable, in such a manner that the successive circuits are tunable to substantially the same frequency at the high frequency end of the tuning range, such as at 1500 kc., and are gradually mistuned' to a greater degree increasingly as the tuning approaches the low frequency end of the tuning range such as 600 kc., for example, the tuning being such that the selectivity remains substantially uniform over the entire tuning range.

It has been found that in case the inductive reactance elements are provided by the secondaries oi tunable radio frequency transformers, the desired mistuning may be obtained also by providing one primary in association with one of the secondaries with sufllcient inductance to resonate at or below the tuning range desired, while the primary Winding associated with another of. the secondary windings is tuned to resonate at or above the desired tuning range.

The invention will. however, be better under-- stood from thefollowing description when con .sidered in connection with the accompanying the operation of the circuits of Fig. 1.

. Referring to Fig. 1, 5 is the R.-F. amplifier tube and 6 is the first detector tube of a superheterodynereceiver. of the type arranged for multiple wave band tuning. Only the A or broadcastband circuits are shown, for the sake of simplifying the drawings. Certain of the waveband-change switch-sections are indicated at i, 8 and 9, since they are included in the grid circuits of the tubes 5 and 6 and the A band tuning circuits.

The signal input grid It of the R.-F. amplifier tube 5 is connected througha suitable grid resistor It with a bias potential supply lead 12, and through a coupling capacitor 13 to a switch contact I l of the section 1, from which circuit connection is made through a movable connecting link is and acontact l B with the high potential end of the A band tuning inductance 17. This inductance is provided by the secondary of a tunable input transformer and receives signal input energy from a high inductance primary Winding it connected with an antenna or signal source IS.

The winding l8 resonates at a frequency below the broadcast band, while the secondary i1 is tunable through the broadcast band by a shunt variable tuning capacitor 20. The latter is one section of a variable tuning capacitor having additional sections 2! and 22 for tuning the firs detector and oscillator respectively.

The tuning capacitor is of the grounded rotor' type as indicated, and is arranged for unicontrol of the various sections as indicated by the dotted connecting line 23. Each capacitor section is provided with a shunt trimmer capacitor indicated at M. The capacitor section '29 and the shunt trimmer capacitor therefor are connected through a lead 25 with the input circuit of the tube& as indicated at 26. Since the secondary i1 is grounded as indicated at 2?, this connection places the tuning capacitor 20 in shunt with the secondary H. I

In a similar manner, the signal input grid 28 of the detector or mixer output 5 is connected through a grid resistor 29 with the lead l2 and also is connected through a coupling capacitor 38 with a contact 3! of the switch section 9 from which the input circuit is completed through a movable connector 32 in the switch, a second con- 'tact33 and a lead 3 5 to the high potential end of. the A band tuning inductance 35 for the detector. This is alsov connected to ground at its low potential side, as indicated at 36.

The variable tuning capacitor section 2| and the trimmer capacitor therefore are connected in shunt with the inductance 35 through a lead.

31 at a point indicated at 38. The inductance '35 is also provided with a fixed shunt capacitor resonates at a frequency below the broadcast band to which the secondary 35 is jointly tunable along with the secondary ll.

As the oscillator circuit is not part of the present invention, the same is. not shown except for the tuning capacitor 22 therefor and the grid input circuit 43 for the detector or mixer tube in connection with the oscillation input grid M. Signals at the intermediate frequency are derived from the detector 5 through the output circuit 45 and applied to the usual R.-F. amplifier (not shown), through a coupling transformer indicated at (it. As the amplifier does not concern the present invention, the circuits thereof are also omitted from the drawings for the sake of simplifying the same.

The output plate circuit of the R.-F. amplifier 5 is coupled to a trap circuit Ell comprising a variable inductance 5| and a capacitor 52 in series, shunted by a resistor 53 and a capacitor 54. The trap circuit is grounded as indicated at 55 and the high potential end thereof is connected through a contact 55 on the switch section 9, a movable link 51 and a second contact 58 with a coupling capacitor 59 to the plate circuit. This places the trap circuit in shunt with the primary winding 4! since the plate supply lead indicated at Bil is by-passed to ground 6! through a by-pass capacitor 62.

The trap circuit is tunable to an interfering signal by means of a variable magnetite core.

indicated at 63, The use of the trap circuit in the primary circuit ofthe R.-F. amplifier tube has been found to be effective to reduce interference when utilizing band-pass tuning means providing uniform selectivitythroughout the tuning range and is more eifective for this purpose than if placed in the antenna circuit. As normally provided in the circuit the trap is tuned to the intermediate frequency to prevent interference from signals in the intermediate fre-' quency range. The resistor 53 is used to damp the parallel resonance peak occurring above the series resonance peak, and does not broaden the I arrangement shown, it will be seenthat the trap circuit is interposed in the signal tuning system betweenthe two variably tunable circuits, in shunt withthe' primary of, the interstage R..-F.

transformer. I i i As previouslyreferred to, the coils or inductances ll and 35,111 the cascaded signal receiving circuits, increase in selectivity as the frequency is reduced. Therefore, to provide a constant band width, the circuits are mistuned at the low frequency end of the tuning range. For the broadcastband this may be in the neighborhood of 600 kc.v This is done by increasing the inductance of one coil over the other, for example, by winding more turnson one coil than on the other. The difference between the resonance frequencies of 'the'circuits contained in thecoils H and at substantially 600 kc. maybe'made approximately 10 kc. Since the amplification is normally higher at the low frequency end of the range due to the lowfrequency primary, the

mistuning tends to cause the overall sensitivity tobe more constant in tuning from the high frequency to the low frequency end of the range.

Referring'to Fig. 2, if the inductances I1 and 35 are aligned to thesame frequency, the overall response of the tuning system will be as shown by the curves E55, 56 and 6'1 corresponding to 600,

1,000 and 1500 kc, respectively. If the maximum modulation frequency which is desired is indicated by the frequency range a, it will be seen that considerable attenuation will be obtained,

particularly at the low frequency end of thetuning range, resulting the loss of fidelity;

Howevenif equal variable capacitors in each circuit such as the capacitors 20 and 21 are used, andif the same coils or inductances are used except that they are'adjusted in inductance so that the resonance frequencies are 10 kc. apart and aligned at 1500 kc, it has beenfound that the width of the modulation frequency band a will be substantially constant throughout the tuning range, as is indicated in Fig. 3 in connection with the curves 68, 09 and 10 corresponding respectively to the tuning of the circuits at 600,

1,000 and 1,500 kc., respectively. Itwill be noted that the desired modulation band width a falls well within the three curves without appreciable attenuation throughout the tuning range.

The efiect of mistuning may be more readily seen by referring to Fig. 4, wherein the tuning response .of the circuit, the input circuit including coils ii, is indicated by the curve H and the tuning response of" the circuit containing the coil 35 is indicated by the curve 12. However,

either of the circuits may be tuned to the higher I frequency. In. the present example, one circuit end of the tuning range, such as at 1,500 kc., by

means of the shunttrimmer capacitors 2 The capacitors 20 and 2! may be equal, that is, substantially identical in capacity and capacity range variation, thereby simplifying the construction of the receiver and lowering the cost.

ence in While this represents a departure from previous practice and substantially a reversal of cer tain methods previously employed in connection with radio receiving systems for providing Variations in selectivity withtuning, it has beenfound adapted for high fidelity reproduction. It has the advantage that the variable tuning capacitors employed may be made alike-and uni-controlled,

while the circuit changes referred to include the relative values of the tuning inductances in the various successive cascaded circuits. 1

For the broadcast tuning range of the present example, the inductance in one circuit may be 4.

percent greater than in the other at 600 kc.-be-

tween the two cascaded tunable circuits, such as R..-F. and detector circuits of the usual superheteradyne receiving system to provide a differresonance at the low frequency end of the broadcast range of substantially 10kc,

At the righ frequency end'of the tuning range, the two circuitsreferred' to are aligned substantially to the same frequency by the trimmer capacitors and the divergence in resonance between the two cascaded circuits increases from i to provide a substantially constant band Width 7 i in the tuning response of radio receiving systems the high frequency end of the tuning range to- H ward thelow frequency end thereof as the tuning ofthe receiving system is Varied.

Mistuning of the circuits at'the low frequency end of the tuning range compensates for the tendency for the selectivity to increase as tlrie frequency to which the circuits are tuned is re:

the tuning range, the mistuning tends to provide a more constant gain or sensitivity for the receiv-Z ing system. However, the low frequency or high inductance primary windings tend to increase the low frequency gain and prevent the mistuning from excessively reducing the gain at the low frequency end of the range, 1

While theinvention has been described in connection with a radiotuning system having a multiple unit variable capacitor, as has hereinbefore been referred to, the tuningjmay be efiected by variable inductance elements in connection with fixed capacitor elements, inwhich case the inductance elements H and 35 may have equal inductance values while the capacitors 20 and 2i have differing capacity values to provide misalignmentof the tuning at the low frequency end of the tuning range as the inductance, ele-' ments are varied conjointly-after the manner of thecapacitors 20, 2E and 22, by common control signal, and coniointly selecting the amplified siga nal at a frequency which differs from the first named frequency by an increasing amount as signals of lower frequencies are selected within said frequency range.

2. The method of receiving radio signals which comprises tuning to select signals throughout a predetermined frequency range at the same resonance frequency in a plurality of cascaded circuits at the high frequency end of said range and at differing resonance frequencies in sa i.d circuits at the low frequency end of the range,

and causing the difference in said resonance frequencies to be reduced as the tuning approaches the high frequency end of the tuning range.

3. In a radio receiving system, means for tuning said system through a predetermined frequency range with substantially uniform selectivity, comprising two tunable signal circuits connected in cascaded relation to each other and comprising inductance and variable capacitor elements, said capacitor and inductance elements providing inductive and capacitor tuning reactances in said circuits, one of said reactances of like character in each circuit being conjointly variable through the same reactance connection 15;

range and the other of said reactances in each circuit being of such differing fixed reactance value that said circuits are mis-tuned increasingly toward the low frequency end of the tuning range with respect to a desired signal frequency, and means-for causing said circuits to reach substantial alignment at a desired signal frequency at the high frequency end of the tuning range.

4. In a radio receiving system, means for tuning said system through a predetermined frequency range with substantially uniform selectivity, comprising-two tunable signal circuits connected in cascaded relation to each other and comprising inductance and variable capacitor elements, said capacitor elements having substantially the samecapacity variation range and said inductance elements having differing inductance values,whereby said circuits are mistuned increasingly toward the low frequency end of the tuning range with respect to a desired signal frequency, and means for causing said circuits to reach substantial alignment at a desired signal frequency at the high frequency end of the tuning range.

5. In a radio receiving system comprising two tunable signal receiving circuits arranged in cascaded relation to each other, the combination of unequal inductance elements in said circuits and substantially equal tuning capacitors in shunt with said inductance elements, and the difference in inductance between said inductance elements being such that a gradual departure between the resonance frequencies of said circuits is effected as the tuning approaches the low frequency end of the tuning range.

6. In a radio receiving system comprising two tunable signal receiving circuits arranged in cascaded relation to each other, the combination of unequal inductance elements in said circuits and substantially equal tuning capacitors in shunt with said inductance elements, a trimmer capacitor for each of said first-named capacities for aligning said circuits to the same resonance frequency at the high frequency end of the tuning range thereof, and the difference in inductance between said inductance elements being such that a gradual departure between the resonance frequencies of said circuits is effected as the tuning approaches the low frequency end of the tuning range.

7.- In a radio receiving system, the combination of means providing a tunable signal input circuit comprising an inductance element and a shunt variable tuning capacitor therefor, a coupling winding for said inductance element for applying signals to said input circuit resonant at a frequency of the order of the lower end of the tuning range of said circuit, a second signal input circuit comprising a second inductance element and a second shunt variable tuning capacitor therefor, means for conveying signals from said first-named circuit to the secondnamed circuit including an amplifier having an output circuit, a winding in said output circuit coupled to the inductance element of said second-named circuit and resonating at a frequency below the tuning range of said second-named circuit, means for conjointly moving said tuning capacitors to tune said circuits through the substantially same frequency range, the inductance element in one circuit having a percentage greater inductance than that in the other circuit by an amount sufficient to cause said circuits to gradually depart from resonance to a desired signal increasingly toward the low frequency en of the tuning range.

8. In a tunable radio receiving system, the

combination of a tunable signal input transformer and an interstage radio frequency transformer, said transformers having secondary windings of unequal inductance value, means for tuning said secondary windings through a predetermined frequency range comprising substantially equal capacity variable tuning capacitors, one connected in shunt with each of said secondary windings, means for conjointly controlling said tuning capacitors for tuning said windings simultaneously through the same frequency range, the inductance of one secondary Winding being greater than that of the other by an amount sufficient to extend the band width of the receiving system at the low frequency end of the tuning range to equal at least the band Width of the tuning system, at the high frequency end of the tuning range.

9. In a tunable radio receivingsystem, the combination of a tunable signal input transformer and an interstage radio frequency transformer, said transformers having secondary windings of unequal inductance value and primary windings resonant at a frequency below the tuning range of said system, means for tuning said secondary windings through a predetermined frequency range comprising substantially equal capacity variable tuning capacitors, one connected in shunt with each of said secondary windings, means for conjointly controlling said tuning capacitors for tuning, said windings simultaneously through the same frequency range, the inductance of one secondary winding being greater than that of the other by an amount suflicient to extend the band width of the receiving system at the low frequency end of the tuning range to equal at least the band width of the tuning system at the high frequency end of the tuning range, and means including a shunt trimmer capacitor for each of said variable tuning capacitors for aligning the tuning of said circuits to the same frequency at the high frequency end of the tuning range.

10. In a radio signal conveying system, the combination of means for tuning said system through a predetermined frequency range in cluding a pair of tunable signal circuits arranged in cascade relation to each other, and means in each of said circuits cooperating to provide substantially a constant band width in the frequency response of said system throughout the tuning range thereof, said last named means including an inductive rectance element and a capacitive reactance element in each of said circuits, like reactance elements of one type in each of said circuits being substantially equal and conjointly variable, and the reactance elements of the opposite type being unequal and fixed to values for effecting resonance in the said circuits at subllpIn a radio signal conveying system, the

combination of a tunable signal input trans former and an interstage signal conveying transformer, said transformers having secondary windings and primary, windings associated therewith, one of said primary windings being resonant at a frequency of the order of the low frequency end of the tuning range of the secondary wind.-

ings, a shunt tuning capacitor for each of said secondary windings, said tuning capacitors being of substantially equal capacity value and tuning variation range, and means for conjointly controlling said tuning capacitors for tuning said windings simultaneously through the same frequency range, the resonance frequencies'of the primary windings being such that the over-all band width of the frequency response of the con: jointly tunable circuits at the low frequency end of the tuning range is substantially equal to the over-all band width of the circuits at the high frequency end of the tuning range.

12. In a radio signal conveying system, the combination of means for tuning said system through a predetermined frequency range include ing a pair of tunable signal circuits arranged in cascade relation tofeach other, and means in each of said circuits cooperating to provide substantially a constant band width in the frequency response of said systemflthroughout the tuning range thereof, said last namedmeans including unequal inductance elements in said circuits for causing divergence in the resonance frequencies elementin each of said circuits, and a variable.

tuning capacitor in shunt therewith, said inductance elements being of unequal inductance value, and said tuning capacitors being of substantially equal capacity'value and capacity variation rangeconjointly operable in unison.

14. In a superheterodyne receiving system, the combination with a radio frequency amplifier and a first detector, of tunable signal input circuits for said amplifier anddetector, an inductance element in each of said circuits, a variable tuning capacitor in shunt therewith, said inductance winding.

said tuning capacitors beingof substantially equal capacity value and capacity variation range conjointly operable in unison, an output circuit for said amplifier, and a tunable wave trap in said;

circuit. l

15. In a superheterodyne receiving system, the

combination with a radio frequency amplifier and afirst detector, of tunable signal input circuits for said amplifier and detector, an inductance element in each of said circuits and a variable tuning capacitor in shunt therewith, said elements being of unequal inductance value, and

inductance elements being of unequal inductance value, and-said tuning capacitors being of substantially equal capacity value and capacity variation range, an output circuit for the said amplifier including a primary winding resonant at a frequency below the tuning rangefof said system and coupled to the inductance element of said detector circuit, and an adjustable wave,

trap circuit connected in shunt with said primary 16. In a superheterodyne receiving system, the combination with i a radio frequency amplifier and a first detector, of tunablesignal input circuits for 'said amplifier and detector, an inductance element in each of said circuits and a variable tuning capacitor in shunttherewith, said inductance elements being of unequal inductance value, and said tuning capacitors being of equal capacity value and capacity variation range, an

output circuitfor the said amplifier including a i primary winding resonant at a frequencybelow the tuningrange of said system and coupled to the inductance elementof said detector circuit,

and an adjustable wave trap circuit connectedin' shunt withsaid primary winding, said wave trap circuit including an inductance having amovable magnetite core andqa tuning capacitor in series therewith, a shunt resistor for limiting the impedance of said series connected inductance and capacitor, and means for connecting said wave trap in circuit with said amplifier.

1'7. In a superheterodyne receiver, alradiofrequency and a detector circuit, means for conjointly tuning saidcircuits throughout the same i frequency range, comprising an inductance ele-n ment in each circuit and a shunt variable tuning capacitor therefor, means for aligning said circuits to the same frequency adjacent the high frequency end of the tuning range'of said cir 

